This invention relates generally to synchronous speed reluctance machines, and more particularly, to the manufacture of rotors for a reluctance type machine.
It is to be understood that the present invention relates to generators as well as to motors, however, to simplify the description that follows, a motor will be described with the understanding that the invention also relates to generators. With this understanding, a synchronous reluctance motor is a synchronous machine having a stator with poly-phase windings forming a plurality of poles that are similar to those of induction motors. The synchronous reluctance motor further includes a rotor that does not use windings or permanent magnets, but does have the same number of poles as the stator. By providing a rotating field in the stator windings, a magnetomotive force acts upon the rotor resulting in the rotor being driven at a synchronous speed proportional to the rotating field in the stator.
Synchronous reluctance rotors typically include a plurality of rotor sections formed of stacked alternating magnetic and non-magnetic laminations secured to a unitary core. The core has a central axial bore for receiving a shaft. The laminations are inserted between radially extending arms of the core, which are formed with a smooth, arcuate recess there between. The laminations are secured in the recesses by means of radial fasteners that secure radially-opposing rotor sections to the core. The rotor sections are also secured together by end caps and axial fasteners. The end caps are cup-shaped members with an axially extending outer rim disposed about the outermost periphery of the laminations. The axial fasteners extend through the end caps and core to secure the end caps to the rotor. The rotor laminations may also be bonded to one another and to the core using an epoxy or other adhesive material. A full description of the laminated stack type synchronous reluctance rotors is disclosed in U.S. Pat. No. 5,296,773.
Methods of building rotors for synchronous reluctance machines include stacked axial laminations and stamped radial laminations. Stacked axial lamination synchronous reluctance rotors are structurally weak due to problems associated both with the fastening and with shifting of the laminations during operation of their many circumferentially discontinuous components. This results in a drastically lower top speed. Moreover, the individual lamination segments have to be independently fabricated and meticulously assembled in a time consuming and costly process. Stamped radial laminations require structural support material at the ends and in the middle of magnetic insulation slots. This results in both structural weakness due to the small slot supports and reduced output power due to magnetic flux leakage through the slot supports. Radial laminations also have a substantially lower top speed than comparably sized induction motors.
There is thus a need to develop a synchronous reluctance machine that exhibits high power density and efficiency and high speed rotating capability similar or better than that of comparably sized induction motors, and preferably that may be produced at a lower cost than that of induction motors.
The present invention provides a composite powder metal disk for a rotor assembly in a synchronous reluctance machine, the disk having alternating regions of magnetically conducting powder metal and magnetically non-conducting powder metal compacted and sintered to a high density. In a further embodiment, a rotor assembly is provided having a plurality of the composite powder metal disks axially stacked along and mounted to a shaft. There is further provided a method of making such a composite powder metal disk and rotor assembly in which a die is filled according to the desired alternating magnetic pattern, followed by pressing the powder metal and sintering the compacted powder to achieve a high density composite powder metal disk of high structural stability. These disks are then stacked axially along a shaft with their magnetic patterns aligned to form the powder metal rotor assembly. A synchronous reluctance machine incorporating the powder metal rotor assembly of the present invention exhibits power density and efficiency comparable to induction motors and improved high speed rotating capability, yet may be produced at a lower cost.